One of the principal functions of a radio frequency (RF) transmitter is to translate its modulated RF carrier to higher RF power, so that the modulated RF carrier can then be radiated over the air and successfully received by a remote receiver. Converting the RF carrier to higher RF power is the responsibility of the RF transmitter's power amplifier (PA). Because the PA is typically the component in the RF transmitter that consumes the most energy, one of the primary goals typically involved in the design of an RF transmitter is making the PA operate as efficiently as possible.
Designing a PA that can operate with high efficiency presents a difficult challenge. Many modern communications systems employ complex modulation schemes in which both the magnitude and the angle (i.e., phase or frequency) of an RF carrier are modulated to convey information. By modulating both the magnitude and angle of the RF carrier, rather than just the magnitude or just the angle of the RF carrier, the RF spectrum can be used more efficiently. Unfortunately, designing a PA that is capable of converting the resulting non-constant envelope RF carrier to higher power with high efficiency is difficult, particularly when the non-constant envelope RF carrier has a high peak-to-average ratio (PAR).
One well-known and increasingly utilized approach to achieving high PA efficiency is to employ a type of RF transmitter known as a polar modulation transmitter. FIG. 1 is a simplified drawing of a polar modulation transmitter 100. The polar modulation transmitter 100 comprises a baseband processor 102, a dynamic power supply (DPS) 104, a phase modulator 106, and a switch-mode PA (SMPA) 108. The baseband processor 102 serves to generate polar-domain amplitude modulation (AM) and phase modulation (PM) signals. The AM signal is applied to the DPS 104, which responds by generating a time-varying DPS voltage VDD(t) that tracks the AM contained in the AM signal. Meanwhile, the phase modulator 106 modulates an RF carrier by the PM contained in the PM signal, producing a constant-envelope phase-modulated RF carrier, which is used to drive the RF input of the SMPA 108. The magnitude of the constant-envelope phase-modulated RF carrier is purposely set high so that the SMPA 108 is overdriven and operates as a switch. As the constant-envelope phase-modulated RF carrier switches the SMPA 108 ON and OFF in accordance with the PM, the DPS voltage VDD(t) produced by the DPS 104 is supplied to the power supply port of the SMPA 108, which typically comprises a power field-effect transistor (FET) having a drain that serves as the power supply port. One important property of an SMPA is that the RF output power that it produces depends on the magnitude of its power supply voltage, or, more specifically, on the square of the magnitude of its power supply voltage. This dependency is exploited in the polar modulation transmitter 100 to superimpose (i.e., modulate) the AM contained in the DPS voltage VDD(t) onto the RF output of the SMPA 108 as the SMPA 108 converts the constant-envelope phase-modulated RF carrier to higher RF power. This act of introducing the AM through the drain supply of the SMPA 108 is known as “drain modulation,” and is a capability that avoids having to apply the AM through the RF path of the SMPA 108. Instead, only the constant-envelope phase-modulated RF carrier needs to be applied through the RF path of the SMPA 108.
The ability of the SMPA 108 to operate as a switch and its ability to perform drain modulation make the polar modulation transmitter 100 substantially more energy efficient than a conventional RF transmitter. The conventional RF transmitter employs a linear PA (such as a Class A, AB, or B linear PA), which operates as a controlled current source—not as a switch—and produces RF output power that is independent of, and incapable of being modulated by, its power supply voltage. Consequently, drain modulation cannot be performed in a linear PA and in order for the linear PA to produce a non-constant envelope RF carrier at its output the AM must be passed through the RF input port of the linear PA. Passing the AM through the RF input port of the linear PA requires that RF output power of the linear PA be backed off, such that output signal peaks remain below the linear PA's saturated output power, in order to prevent distortion. This back off requirement, together with the fact that the linear PA operates as a current source and not as a switch results in the conventional linear-PA-based RF transmitter being significantly less efficient compared to a polar modulation transmitter. Therefore, when efficiency is a primary concern, and especially when non-constant envelope signals are involved, the polar modulation transmitter is the better option.
Although the polar modulation transmitter 100 operates with high efficiency, one significant problem that follows from its use is that it can be difficult to reduce the signal envelope of its non-constant envelope RF output to zero or near zero during times when it should. Various wireless communications standards such as, for example, Wideband Code Division Multiple Access (W-CDMA) and Long-Term Evolution (LTE), employ complex modulation schemes in which the magnitude of the signal envelope of an RF carrier must, on certain occasions, be reduced to zero or very near zero. FIGS. 2 and 3 show, for example, waveform snippets of typical signal envelope waveforms observed in communications systems operating in accordance with the W-CDMA air interface (FIG. 2) and the LTE interface (FIG. 3). The waveform snippet in FIG. 2 reveals that although low-magnitude events are rare in the AM in W-CDMA-based communications, as indicated by the circled low-magnitude event 202, they nevertheless occur. The waveform snippet in FIG. 3 shows that low-magnitude events also occur in the AM in LTE-based communications, and tend to occur more frequently. The polar modulation transmitter 100 has difficulty reproducing these low-magnitude events in the signal envelope at the RF output of the RFPA 614 for two primary reasons. First, the output stage power transistor of the SMPA 108, which is typically a FET, has a gate-to-drain capacitor Cgd that provides a parasitic leakage path through which the constant-envelope phase-modulated RF carrier, which, as explained above serves as the RF switch drive signal for the SMPA 108, can leak to the output of the SMPA 108. FIG. 4 is a plot of the RF output power of a typical SMPA showing this leakage effect. The RF output power of the SMPA is plotted in decibels (relative to the average output power of the SMPA (i.e., dBr)) as a function of the normalized DPS output voltage. Ideally, the RF output power of the SMPA is proportional to the square of the magnitude of the DPS voltage VDD(t), and for all magnitudes of VDD(t). This ideal characteristic is shown in FIG. 4 by the “square law” straight line. At higher magnitudes of VDD(t), for example when the normalized DPS voltage in the plot is greater than 0.1, leakage is not seen to be a major concern. However, at lower magnitudes of VDD(t), for example when the normalized DPS voltage drops below 0.01, the leaked RF switch drive signal begins to dominate the RF output of the SMPA and prevents the magnitude of the signal envelope of the RF output from dropping down to lower magnitudes, as it should.
A second reason it can be difficult for the polar modulation transmitter 100 to reduce the magnitude of the signal envelope of its RF output to zero or near zero when the intended AM dictates that is should relates to bandwidth handling limitations of the DPS 104. As can be seen in FIGS. 2 and 3, the signal envelopes in state-of-the-art communications signals tend to inflect very sharply during occurrences of low-magnitude events. In some cases the bandwidth handling capability of the DPS 104 will be insufficient for the DPS 104 to accurately track these sharply inflecting low-magnitude events. Consequently, rather that the DPS 104 producing a DPS voltage VDD(t) like the desired DPS voltage depicted in FIG. 5A, the DPS 104 produces a DPS voltage that more resembles the DPS voltage depicted in FIG. 5B. The inability of the DPS 104 to reproduce low-magnitude events contained in the original AM at its output thus results in the SMPA 108 also producing an RF output with a signal envelope that does not reduce to zero or near zero when it should.
The inability of the polar modulation transmitter 100 to faithfully produce low-magnitude events at its output is undesirable, whether the inability is attributable to leakage of the phase-modulated RF switch drive signal through the leakage path of the output stage power transistor the SMPA 108, is attributable to bandwidth handling limitations of the DPS 104, or is attributable to a combination of both problems. The inability to faithfully produce the low-magnitude events can lead to errors at the receiving end of the communications system, and can make it difficult, and sometimes even impossible, to comply with signal accuracy requirements (for example, maximum permissible error vector magnitude (EVM)) set forth in the controlling communications standard. The present invention addresses and provides solutions to these problems.